Pokémon Forms

Pokémon Forms are variations between individual Pokémon, these variations may be different in types, ability, stats, or learnsets sometimes forms are purely aesthetic. Pokémon Forms are first introduced in Generation II with Unown but purely aesthetic. In Generation III some Pokémon forms begin to bear differences in types, ability, stats, or learnsets with Castform (weathers effecting its typing) and Deoxys (stats and learnsets variations among forms however Deoxys' forms are version locked until Generation IV).

Venusaur and Blastoise

Rayquaza and Mismagius

Dialga and Palkia

Legendary Birds

Pseudo-forms

Pseudo-forms are not official Pokémon Forms but rather variants changes that are purely aesthetic or it can change effect typing and attack's typing. There are currently two Pokémon with Pseudo-forms: Arceus and Genesect.

Arceus

Arceus has seventeen different type variatons, which correspend to the seventeen different types.
Arceus's transformation between these forms is brought about by its held item: if it is holding a Plate, it draws its type from that Plate as stated in Sinnoh's mythology, while remaining Normal-type when not holding a Plate.
In the Generation IV games, there is dummy data and a sprite programmed for a ???-type Arceus, but it cannot be obtained as there is no ???-type Plate. This data no longer exists in Generation V due to the removal of the ??? type.
While its base stats remain constant despite its changing type, its Pokéathlon performance changes slightly. Additionally, it can learn Draco Meteor only while holding the Draco Plate.

Genesect

In the games, the Drive on Genesect's back is visibly depicted on its sprite depending on which Drive it holds. However, this does not change Genesect's type, but rather the type of the move Techno Blast. If it lacks a Drive, it will be Normal-type and show a dark yellow Drive connected to it. The four Drives that can be equipped are the Shock, Burn, Chill, and Douse Drive, which make Techno Blast Electric, Fire, Ice, and Water-type, respectively. Otherwise, the change is purely cosmetic.

Gender differences

These variations are purely aesthetic between the Pokémon gender and they are not classify as forms (with the exception of Nidoran♀ and Nidoran♂ groups which are two different species of Pokémon).

Shiny Pokémon

Shiny Pokémon are colored in a different coloration than it normal usual colors also Shiny Pokémon colorations are purely aesthetic however these Pokémon's summary does have a special icon indicating its special coloration status.